DEV: correct flaky test relying on possibly valid user id

This test lasted about 7 years prior to it becoming flaky.

Today ... for whatever reason the test suite created 100 users
prior to running this spec. So the new user becomes user id 101

And... lo and behold:

```

  1) PostSerializer a hidden post with add_raw enabled a hidden post shows the raw post only if authorized to see it
     Failure/Error: expect(serialized_post_for_user(Fabricate(:user))[:raw]).to eq(nil)

       expected: nil
            got: "Raw contents of the post."

       (compared using ==)
     # ./spec/serializers/post_serializer_spec.rb:127:in `block (4 levels) in <main>'
```

spec/serializers/post_serializer_spec.rb

@@ -106,7 +106,7 @@ describe PostSerializer do
   end
 
   context "a hidden post with add_raw enabled" do
-    let(:user) { Fabricate.build(:user, id: 101) }
+    let(:user) { Fabricate.build(:user, id: -99999) }
     let(:raw)  { "Raw contents of the post." }
 
     context "a public post" do